In the LSI for a large-scale information communication equipment and the LSI for a mobile communication equipment, the reduction in power and area has been an important task to be achieved. One of the solutions thereof includes the miniaturization of the process, and it is advantageous also in the low-voltage operation. However, the design conditions become stricter in terms of characteristics of an analog circuit such as dynamic range and variation. Therefore, a digital circuit advantageous in the miniaturization of process is used as an equivalent circuit of an analog circuit. The digital equivalent circuit compensates for performance deteriorations of the analog circuit such as the element variation due to the area reduction and the reduction of the dynamic range due to the low-voltage operation (digital assist).
For example, in the control methods described in Japanese Patent Application Laid-Open Publication No. 4-273618 (Patent Document 1) and U.S. Pat. No. 7,408,416 (Patent Document 2) , a circuit is added in a loop of a phase locked loop circuit to control a control voltage input to a voltage controlled oscillator, thereby achieving the desired characteristics.